The route of the main east-west Austrian rail link runs over the Arlberg Mountain. The original link was built more than 100 years ago as a single-track line. As part of the TransEuropean Network (TEN), this link will be enlarged to become a double-track line using state-of-the-art technology. The St Anton/Arlberg section of the link has been given priority because of the world skiing championship, which will take place in February 2001.
It is planned to renew the alignment over 6km from the centre to the outskirts of St Anton. Because of this realignment, it will be necessary to build bridges and a 600m long avalanche gallery; reprofile a 150m section of the 10km long existing Arlberg Tunnel, built in the last century; and construct the 1743m long Wolfsgruben Tunnel, including a 175m long rescue adit.
The geological and geomechanical conditions of the Wolfsgruben Tunnel are problematical. In both portal areas, the tunnel passes through a zone containing 50m of talus deposits and 200m of moraine debris. The alignment is mainly in the mica schist and gneiss schist. In addition, there are indications of fractured zones and a fault line.
The project is engineered by the Austrian Federal Railways (ÖBB), which is also the client. Final planning will be carried out by Austrian engineering and consulting firms. The project is divided into several sections. The contract for the Wolfsgruben Tunnel has been awarded to a JV of Stuag; Porr–Asdag; Alpine Bau; Ilbau; and Jaeger. The driving force for the execution of the works is the time schedule. The completion date is the beginning of the alpine skiing world championship in February 2001.
Contracts were awarded in June 1998. All construction works, final railway works and landscaping has to be carried out within two years. The main construction works were predicted to take 18 months.
This short construction period will require the Wolfsgruben Tunnel to be excavated from four faces, with three faces being excavated simultaneously. The main tunnel has a full face area of 120-130m², the rescue adit 55m². It was decided to excavate the main tunnel from both sides of the portal and the adit at the same time. After the adit was finished, tunnelling followed the direction of the main tunnel. Because of the different ground conditions, the length of each portion of tunnel is different.
Risk of avalanches
Excavation started in September 1998 at the east portal and on the rescue adit, with predicted lengths of 300m and 175m respectively. The tunnel sections from the end of the rescue adit are 500m to the east and 660m to the west; a short 66m section had to be driven from the western portal. Because of the risk of avalanches and the amount of open cut, it was not possible to start tunnelling from the western portal until May this year.
At end of October 1998, the rescue adit was finished and excavation of the main tunnel started in both directions. All work had to be stopped for two weeks in February 1999 owing to the risk of avalanches. However, tunnel excavation was finished by the beginning of July 1999.
The Wolfsgruben Tunnel has been constructed according to NATM principles, strictly following the directions of the client, in accordance with the support methods calculated for the different rock conditions. Top headings were followed by bench and invert excavation within defined sections behind the face.
The main installations, i.e. the workshops; transformer station; compressors; wastewater station; batching plant; and the contractor’s and engineer’s offices, were located in front of the portal of the rescue adit because of the limited area around the portals of the main tunnel.
Composition of the shotcrete
For the shotcrete a prefabricated mixture of oven dried aggregates and cement free of alkaline was used. It was stored in silos close to the portal, transported to the face area and then transported by silo dumpers to the shotcrete pumps.
Tunnel equipment used included: a boomer with two arms; a 130kW excavator for profiling; a 170kW wheeled loader with a 3.6m³ bucket; two mucking dumpers; a lifting platform; a silo dumper; and a shotcrete unit. Ventilation was carried out by blowing in fresh air. In the portal area, two compressors with 26m³/min by 8 bar at each were installed for compressed air supply. For profile marking on the face, a motor laser was used, programmed by the surveyor for each single round face. This increased accuracy and reduced surveying expense.
The rescue adit was driven without any serious problems. It was supported with shotcrete, a single layer of wire mesh and rockbolts. The ventilation tubes were dimensioned both for adit excavation and for main tunnel driving.
It was expected that the talus deposits and moraine debris close to the portal area of the main tunnel subjected would move downstream, so additional support, such as grouted umbrellas and pile foundations for the downstream side of the tunnel, were required.
Tunnelling from the east was more complicated. The portal was placed15m above the ground level on a steep slope with serious risk of landslides. In addition, the inclination of the longitudinal tunnel axis to the direction of the slope was not advantageous for tunnelling. For preliminary support, a section of open tunnelling with an overlying embankment as ballast and a single grouted umbrella had to be constructed before excavation could begin.
Grouted umbrella
The grouted umbrella was designed with drilling lengths of 20m having a 2m overlap. After the preliminary support was completed, excavation started with a top heading. After approximately 50m, cracking of the shotcrete was reported and measurements showed that the slope had started to move downwards and the excavated crown collapsed. Additional support comprising pile foundations underneath the downstream base of the crown and an arrangement of 18m long rockbolts upstream were essential and allowed excavation of the top heading to continue.
The following days showed that the rock situation in the top heading was worse than predicted by the geological report. The face was much more fractured and jointed so that, for the whole length of the east section, forepoling was necessary. This resulted in a serious discussion concerning the time schedule. The breakthrough point in this section was moved approximately 80m to the east to adhere to the timetable. To support the top heading, piling (length 3-4m), several layers of 250mm shotcrete, a double layer of wire mesh, rockbolts and steel ribs were applied. The maximum length of one round was limited to 1m.
Originally, it was planned to excavate the bench to invert level and, if necessary, to profile an invert arch in certain sections only. Because of the additional knowledge of the ground derived from excavation of the top heading, the whole length of this section was driven with an invert arch excavation with reinforced shotcrete support over the whole profile.
Top heading damage
Main tunnelling from the end of the adit in an easterly direction began as scheduled but the ground became worse as tunnelling progressed, resulting in limited damage to the top heading. With additional support in the form of longer piling and shotcrete, the collapse was restored. Tunnelling was carried out as described above, except that the invert arch was placed in limited areas only. The rescue adit was used for ventilation, mucking and supply.
The tunnelling section from the end of the adit to the west was carried out without any serious problems so that it was possible to increase the length of a single round to 3m for the top heading. The procedure was as described above. Instead of invert arches, a concreted base slab was constructed. Although the rock was in good condition, it was necessary to install support with steel ribs over the whole length of the top heading of this section too. The rescue adit was used for ventilation, mucking and supply.
The 66m section from the western portal was considered the most difficult section of the Wolfsgruben Tunnel. Because the St Anton railway station is located at the end of the tunnel, it was necessary to design the transition from a double-tracked line to a four-track railway within the tunnel.
The portal is located in a section of deep open cut in talus deposits and moraine debris. A 60m long grouted umbrella, divided into three parts, comprised the preliminary support. The first umbrella was constructed from the portal and was longer than the following two sections so that tunnelling at the start was not interrupted.
The umbrellas comprised 76mm diameter perforated steel pipes and cement grout was used to stabilise the surrounding rock. Although the cross section exceeded 180m², it was possible to excavate the top heading in full face. Support consisted of 350mm thick shotcrete, two layers of wire mesh, lattice girders and self drilling grouted rockbolts 10m long upstream and 6m downstream.
Environment
In modern civil constructions, environmental constraints present a challenge to designers and contractors. Standards are now in force to limit noise and emissions from tunnel plant, and wastewater must be treated to neutralise the effect of shotcrete and cement additives To eliminate sands and suspended matter, all tunnel wastewater was channelled into two sedimentation basins. The pH value was measured continuously for automatic addition of CO2.
During excavation, extensive measurements were carried out for safe tunnelling, necessitating continuous interpretation and evaluation. As a result, early indication of a top heading collapse In the east section of the Wolfsgruben Tunnel resulted in the decision to use additional support.
After excavation east of the rescue adit, lining operations began. A waterproof membrane was installed next to the excavated surface. The lining is not reinforced, and segment length is 12m. The shuttering is self supporting and can be used both in the regular cross section and in the transition area near the western portal.
Larger cross sections
For the larger cross sections, the shutter can be opened in the crown and additional formwork sections can be added to adapt the shutter. Because of the time schedule, it is necessary to concrete six to seven segments with one shutter each week.
Independently of the disastrous accidents in the Mont Blanc and Tauern Tunnel, many safety measures have been instituted by the designers of the Wolfsgruben Tunnel. The rescue adit can used by all sizes of fire brigade vehicles; in addition, the rail tracks will be designed to be used by rescue and ambulance vehicles. At the end of the adit, a smoke barrier has been installed, and the water supply for extinguishing fires will be secured by a special pipeline.
The Wolfsgruben Tunnel demonstrates the flexibility of the NATM to respond to varying ground conditions. The partnership on site between client, consultants and contractor is based on a high level of cooperation and expertise, resulting in fast decision making so that work is not delayed. Each partner is willing to work with the others to achieve the same goal: to finish the construction works successfully and to a high standard, bringing the project in on time and to budget.
Related Files
Location Map
Cross Section
General Layout
Geological Profile
